The basis of these rules (and policies which become part of rules) is safety. While not every requirement may make sense in every situation, every study on aircraft safety in recent memory, concludes that rigid attention to detailed requirements yields better results in emergency landings and worse.

FA's are not authorized to change the rules because, in one particular situation, it is possible that having the tray table down will not affect passenger safety. And if they were authorized, preparing the cabin for landing would be a 45-minute affair while every know-it-all aeronautical engineer or wannabe argues about not having to do something which ought to just be done.